BUG: filmPyrolysisRadiativeCoupledMixed parallel error

Summary

As reported via @Prashant for EP#2050

Steps to reproduce

See tutorial combustion/fireFoam/LES/flameSpreadWaterSuppressionPanel

• decompose each region (primary, film, pyrolysis) into 8 procs (used scotch)
• parallel failure at runtime due to operating on fields of different sizes